Circuit for simulating vibrato effect by amplitude modulation of tone by sawtooth waveform



H. F. OLSON ETAL CIRCUIT FOR SIMULATING VIBRATO EFFECT BY Feb. 26, 1963AMPLITUDE MoDULATIoN oF TONE BY SAWTOOTH WAVEFORM 3 Sheets-Sheet 1Original Filed Dec. 26, 1951 Feb- 26, 1963 H. F. oLsoN ETAL.

CIRCUIT FCR STMULATINC VIBRATC EFFECT BY AMPLITUDE MCDULATTCN oF TONE BYSAWTOOTH WAVEFORM 3 Sheets-Sheet 2 Original Filed Dec. 26. 1951 i Z Z F/n. .\M was l z l I l l l I l l l I l l I l t Il /3 M@ Feb. 26, 1963 H.F. OLSON ETAL CIRCUIT FOR STMULATING VIBRATo EFFECT BY AMPLITUDEMoDULATIoN oF TONE BT SAWTOOTH WAVEFORM 3 Sheets-Sheet 3 Original FiledDeo. 26. 1951 ATTORNEY United States Patent O ClittCllli'li FR SMULATHNGVHBRAT EFFECT lliY AMEMTUBE MDULATEN F TGNE BY SAW- TTH WMWETERl/i Harryl?. ison, Princeton, and Herbert Bailar, Paimyra, sLIi'., assignors teRadio Corporation of America, a corporation oi Delaware @riginalapplication Dec. 26, 1951i, Ser. No. 268,252, new Patent No. 2,855,816,dated @en 14, 1558. liyided and this application Feb. Z7, 1958, Ser. No.717,853

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This application is a division of our copending application Serial No.263,252, filed December 26, 1951 for Music L.ynthesizen now Patent No.2,855,816, issued October 14, 1958.

The present invention relates to apparatus for producingtonal eiiectsand more particularly to apparatus for controlling changes incharacteristics of a tone, such as may be produ-ced in a musicsynthesizer device.

Apparatus incorporating the present invention will be especially usefulin providing tonal effects involving the auditory sensation of pitch.'Ihe pitch or frequency glide effect known as portamento for changingfrom one produced tone to another may be accomplished with apparatusembodying this invention. Special portamento eilects whereby a desirabletransient may be inserted between tones of two frequencies when a suddenchange is made from one frequency to the other are also possible withapparatus provided in accordance with the invention. The invention isuseful in providing apparatus for producing a vibrato effect. Vibrato isa complex modulation of sound and may involve frequency, amplitude andwaveform modulation of a produced tone.

Tones to which vibrato or portamento effects may be imparted may beproduced in the units of a music synthesizer. A 'music synthesizer is anelectronic system including, in part, a plurality of units forcontrolling the characteristics of the selected tone which are cascadedin a synthesizer channel and which may be controlled in accordance withthe coding of a coded record. More than one synthesizer channel may beused to produce notes alternately or overlapping. In addition, thesynthesizer `channel preferably includes means for connecting in cascadewith the other units thereof, a vibrato unit or a portamento unit orboth. Whether either a vibrato unit or a portamento unit or both are tobe used depends upon the music to be played.

Briefly described, the apparatus provided by the present invention mayinclude, in combination with a music synthesizer channel through which aproduced tone is to be passed, a iirst means for altering the frequencycharacteristics of the tone in response to a control signal, andfrequency control means for generating the control signal and applyingthe signal to the rst means whereby a selected portamento or vibratonote synthesis will be produced.

Accordingly, it is an object of the present invention to provideimproved apparatus for controlling changes in characteristics of a tone.

A further object of the invention is to provide improved apparatus yforobtaining tonal effects involving changes in pitch.

A still yfurther object of the invention is to provide improvedapparatus for obtaining a vibrato effect.

A still further object of the invention is to provide improved apparatusfor obtaining a portamento or frequency glide effect.

A still further object of the present invention is to provide a unit tobe associated with a music synthesizer channel for obtaining a vibratoeffect in the synthesizer output.

3,073,752 Patented Feb. 26, 1963 A still further object of the presentinvention is to provide a unit to be associated with a music synthesizerchannel for obtaining a portamento or frequency glide effect in thesynthesizer output.

Other objects and advantages of the present invention will, of course,become apparent and immediately suggest themselves to those skilled inthe art to which the invention is directed from a reading of thefollowing description in connection with the accompanying drawings inwhich:

FIG. l is a block diagram illustrating parts of a music synthesizerwhich may incorporate portamento and vibrato units provided by theinvention;

PEG. 2 is a block and circuit diagram of the frequency glider orportamento unit shown in FIG. l;

FIG. 2a is a circuit diagram of the frequency meter shown in FIG. 2;

FIGS. 2b and 2c are cir-cuit diagrams of different filters that may beused in the frequency glider unit of FIG. 2;

FiG. 2d is a group of graphs that are referred to in explaining theoperation of the frequency glider;

FIG. 3 is a circuit diagram of the vibrato unit shown in FIG. l; and

FlGS. 3a and 3b are graphs that are referred to in explaining theoperation of the vibrato unit.

In FIG. l, signal frequency sources, such as tuning lforks, areindicated by the `block 5l). Twelve fork sources may be usedcorresponding to the twelve notes in the equally tempered scale. insteadof tuning forks, one may employ tone Wheels, oscillators, or any othersuitable signal source. A binary relay switching arrangement or relaytree 7i is employed for connecting any desired tune fork source to theinput of an octaver 72. By employing a binary switching arrangement incombination with a coded record such as a punched paper roll, it hasbeen possible to reduce very greatly the number of contacts required atthe coded record.

A punched paper roll in indicated at 73. This paper roll passes between`brushes or contact points numbered l to 36, inclusive, and a metalContact and driving roller '7d having sprockets at each end. When a holein the paper roll falls under a brush, a relay coil is connected to saidbrush and is energized to pull down the associated relay armatures. Fromthe foregoing, it will be apparent that by means of a four-hole code, itis possible to connect any of the desired one of the tuning fork sourcesto the octaver 72.

The octaver 72 is a unit for producing a selected note in any desiredoctave within the range of the synthesizer. By way of example, theoctaver may comprise both dividing and multiplying circuits. ie octavermight comprise dividers only or multipliers only, particularly ifdiiferent frequency signal sources are selected. Sa tooth waves ofdifferent frequencies may be obtained at the output terminal of theoctaver '72. The output of the octaver is connected to the octaver relaytree 97.

An envelope Shaper and keyer unit Hl controls the rise, the duration andthe decay of the note or signal passed through the synthesizer channel.It performs a double function; it shapes the signal envelope, and itopens and closes the synthesizer channel. The output of the octaverrelay tree 97 may be supplied directly to the envelope Shaper and keyerunit 121. Alternatively, the output of the octaver relay tree 97 may besupplied through a frequency glider or portamento 1.22 by means of thedouble-pole, double-throw switch 123, when the switch is in the upposition. The portamento unit and its use will be set forth laterherein. When the switch i123 is in the down position, there is a directconnection from the octaver relay tree into the envelope Shaper andkeyer unit.

sheaves f) l The shaper and keyer unit 121 may comprise an amplilier andassociated biasing or unblocking circuits under the contr-ol of thecoded paper roll 73. lt may be mentioned .at this point that in thepreferred operation or the synthesizer, the paper roll is coded so thatall of 4the units of the synthesizer channel are set up before theenvelope Shaper and keyer l2ll causes the channel to be unblocked. inthis way the appearance of relay-clicks, and the like in the finaloutput is avoided.

rThe 4output or" the envelope Shaper and keyer unit i2 is fed to amaster volume control unit l* 3 which is controlled by a relay treel-fi. The relay tree i742 is the same as the relay tree 7l and iscontrolled by Contact points or brushes l5, i6, 17 and i8, associatedwith the coded paper roll.

The output of the master volume control relay tree 17'@ is passedthrough a signal spectrum controller unit lili which is largely for thepurpose of determining the harmonic content of the tone. This is. theunit that determines, to a great extent, whether a tone sounds like thatof a violin or that of a trumpet, for example.

A vibrato unit E92 is shown. This unit may be connected in cascade withthe spectrum controller @l by means of a double-pole, double-throwswitch 193. No1'- mally this would be connected if violin music, for eX-arnple, were being played. The vibrato unit 132 will be discussed later.

The spectrum controller unit 191 comprises filters and networks ofvarious types that `may be selectively connected into the synthesizerchannel by suitable means such as switches or cord connections. In theexample illustrated, a iilter, resonator networks, and compensatingnetworks may be connected in series by means of cord connections.Through this series combination, the output of the master volume controlunit is fed through the spectrum controller ll to a volume equalizerfunit ige.

VThere may be other connections to the volume equalizer unit 194. Thevolume equalizer unit ille preferably comprises an individual vacuumtube arnpliiier for each of the input terminals thereof. The volumeequalizer unit may have several input terminals. The output volume ofeach amplifier of the unit 194 is individually adjustable. In this way,any change in volume caused by the insertion of a different ilter ornetwork in the spectrum controller 191 may be compensated.

A relay tree 1% is controlled by Contact points or brushes S, 9, lll andl1, associated with the coded paper roll. By punching proper codes to bepassed under brushes il, 9, l@ and il, any spectrum controller networkcombination that has been set up may be connected cascade with the restof the synthesizer channel. The output of the relay tree 196 may beconnected through an isolating resistor i9? to a recording equalizernetwork in the event that a record is to be cut, or to an audiofrequency amplifier ltl and loudspeaker i229 in the event that the musicis to be heard directly from the synthesizer.

Two synthesizer channels are desirably employed so that the coded paperrecord can set up one channel While the other channel is in operationand producing a tone. Also, so that one channel can start playing a tonebefore the other channel stops playing the tone.

l shows o second channel which is a duplicate of the one previouslydescribed. Everything is duplicate except for the frequency sources ofblock Sii. Each tuning fork output is connected to an input terminal ofthe relay tree 7l of the first channel and also to a corresponding inputterminal of a relay tree HA of the second channel.

The second synthesizer channel comprises the frequency sources Slicommon to the two channels, the relay tree 71A and octaver 72A, a relaytree 97A, an envelope Shaper and keyer 121A, a master volume controlunit l'fA, a relay tree IMA, a spectrum controller iQlA, a volumeequalizer unit 194A, and a relay tree 196A. The output plied to thereactance tube.

of the second channel is fed through an isolating resistor 223 to theoutput terminal. Thus, the outputs or' the two channels may be suppliedsuccessively or simultaneously to a record cutter or to a loud-speaker.Instead of using the isolating resistors i9? and 225, it may bepreferred to employ a combining amplifier (two tubes with a commonoutput) to which the two channel outputs are applied.

rthe relay trees vof the second channel are controlled 'oy contactpoints or brushes i9 to 36, inclusive, which are associated with thecoded paper roll '73. rthe second channel is controlled by brushes i9 to36 in the sante way that the first channel is controlled by the brushesto I8.

Inspection of FIG. 1 shows that one-half of the paper roll (the leftside as viewed in FIG. l) carries the punched coding for the firstchannel While the other half the roll carries the coding for the secondchannel.

In order to simplify the drawing, the portamento and vibrato units arenot shown associated with the second channel. However, it should beunderstood that ordinarily if such units are connected in the firstchannel, corresponding units are also connected in the second channel.

The various units of the music synthesizer channels, shown in FIG. l, aswell as means for operating and coding the coded paper rolls 73, aredescribed and their operation explained in our copending applicationSerial No. 263,252, iiled December 26, i951 for Music Synthesizer ofwhich the present application is a division, now Patent No. 2,855,816,issued October 14, 1958.

As previously mentioned, it may be desirable to include a frequencyglider or portamento unit in the synthesizer channel. With this unit itis possible to slide from one note to another as is commonly done inplaying a trombone, a violin or a steel guitar.

A frequency glider is represented in FIG. 1 by the block 122 vand isshown in some detail in FIG. 2. Additional details are shown in FIGS.2a, 2b, and 2c.

As shown in FIG. 2, the frequency glider 122 may comprise a` frequencymeter 226 of the type that has a direct current output. The D.C. outputincreases linearly with an increase ink the fundamental frequency of tbeapplied sawtooth wave. The D.C. output preferably is applied through afilter 227 for controlling the rate of -rise or the rise characteristicof the D.C. control signal.

The output of filter 227 is applied to a suitable oscillator 226 whosefrequency is a function of the D.C. control signal. The particularoscillator illustrated is of the beat frequency type comprising avariable frequency oscillator 229 whose frequency is controlled by areactance tube 23E. The D.C. control signal is applied to this reactancetube. The beat frequency oscillator further comprises a stable frequencyoscillator 232 and a detector 2513 to which the outputs of oscillators232 and 229 are applied.

The output of the detector 233 is a sine wave signal having a frequencyequal to the difference in the frequencies of oscillators 229 and 232.This is the desired audio frequency.

Since a sawtooth wave, instead of a sine wave, is desired, the detectoroutput is clipped and differentiated by units 234 and 236, respectively.The differentiated pulses are applied to a sawtooth generator 237 wherethe positive differentiated pulses trigger the generator to produce asawtooth Wave having a fundamental frequency equal to said desired audiofrequency.

The beat frequency oscillator may readily be designed so that thefrequency of its output is a linear function of the direct currentapplied to reactance tube 23E.

In the example illustrated, the direct current output of meter 226increases linearly with increase in applied frequency, and the frequencyof the beat frequency oscillator increases linearly with increase indirect current ap- In operation, when the sawtooth Wave at the frequency`the charges on capacitors 246 and meter input changes from onefrequency to another, the beat frequency oscillator output follows thischange and, likewise, changes from said one frequency to said otherfrequency. This beat frequency oscillator change, however, iscomparatively gradual. The rate of change 1s controlled by the filter227 as discussed hereinafter.

Suitable frequency meters for the present purpose are well known in theart. One `such meter is shown and described on page 95 8 of RadioEugineers Handbook, by T erman. A meter of this type is shown in FIG. 2awhere an output resistor 241 has been substituted for the D.C. meter,and where a cathode follower tube 242 is used t0 feed signal from theoutput resistor 241 to the filter 227.

ln FIG. 2a the tubes 243 and 244 are gas triodes connected in aninverter circuit that comprises capacitors 246 and 247. These capacitorsare alternately charged from the regulated -l-B supply voltage on thepositive and negative halves of the cycle of the applied signal. Eachtime a gas triode becomes conducting, it extinguishes the other gastriode. Thus, assuming tube 244 is conducting, as soon as tube 243 ismade conducting, it puts a short across tube 244 to extinguish it. lTheshorting connection includes a capacitor 248.

rlfhe charging currents of the capacitors 2li@ and 247 flow through thedouble diode 249 and'through the load. resistor 241 as a direct current.Any pulsations in this direct current may be smoothed out by a capacitor(not shown) across the load resistor. Such a capacitor is not providedin the circuit illustrated as sufficient smoothing is provided by thelter 227.

lt Will be noted that, between charging current pulses, 247 leali oir`by way of resistors 251 and 252, respectively.

lf desired, the sawtooth wave may be applied directly to the frequencymeter as indicated in FIG. 2a. However, it may be preferred tirst toclip the sawtooth wave to produce a square wave that is applied to thefrequency meter. Or the sawtooth wave may be passed through a bandpassfilter to obtain substantially a sine wave that is applied to thefrequency meter.

As examples of other suitable frequency meters, reference is made to theaudio frequency meter Type No. 30G-A manufactured by Radio Corporationof America and also the frequency meter shown on page 571 of the text,Electronics Manual for Radio Engineers, by Vin- Zeluif and John Markus,first edition, published by lvtcGraw-Hill Book Company, Incorporated.

Suitable variable frequency oscillators, such as beat frequencyoscillators, are so well known that -it is not necessary to describecircuit details. It may be noted that oscillators are described in thearticle by Charles Travis entitled Automatic Frequency Control, egiuningon page ll25 of the October, 1935, issue of the periodical Proceedingsof the institute of Radio Engineers.

In practice, it is generally found that the frequency glider need nothave an operating range greater than two octaves. Because of thislimited frequency range, there is no difficulty in designing andadjusting the circuit so that the beat frequency oscillator outputfollows the frequency meter input very closely in frequency.

lt may be desirable to provide more than one frequency glider for amusic synthesizer channel, one frequency glider to have arange for thetrombone, another a range for a violin, et cetera. Only one frequencyglider at a time is used. The point is that a two octave range issui'hcient for any one musical instrument.

The frequency glider, of course, must be adjusted initially so that itsoutput has the same frequency as that of the glider input signal. This,for the most part, is done by adjustment of the beat frequencyoscillator. lt may also be done, possibly as a final adjustment, byadjusting a variable tap 253 on the output resistor o-f the cathodefollower tube 242.

The function of the filter 227 will now be described with particularreference to the graphs of FIG. 2d. Filter suitable reactance tubecircuits for variable frequency v at a low frequency.

227 in the form shown in FIG. 2 comprises a variable resistor 254 and avariable inductor 256 in series, and a variable capacitor 257 in shunt.The inductor 256 and capacitor 257 are given such values that theyresonate at some low frequency so that the signal applied to thereactance tube 225i overshoots when gliding from one note to another asshown by the graph 25S in FIG. 2d.

Since the oscillator output frequency follows the overshoot and thefollowing slow oscillation, also shown by graph 25S, the effect is toput a wobble in the tone just before it settles down to a steady tone.This is an effect often found in music. lt may occur as a tromboneplayer starts to sound a new note. The same effect is found in singing.

'The -filter 227 may be made to resonate at about eight cycles persecond, for example. In this case the values may be 400 henrys forinductor 256, one microfarad for capacitor 257, and 100 ohms forresistor 25e. The 100 ohms includes the resistance of coil 256. Theresistor 254 may be adjusted to vary the amount of the over shoot andthe duration of the wobble.

As another example, the filter 227 might be adjusted to resonate at 16cycles per second.

Instead of the filter 227 as shown in FIG. 2, the filter shown in FIG.2b might be used. It comprises a resistor and an inductor in series. ltalso is tuned to resonate ln some cases it Will be preferred to have noovershoo-t. ln such cases a resistor-capacitor filter such as shown inFIG. 2c may be used in place of the filter 227 shown in FIG. 2. Thisfilter comprises a variable series resistor 259 and a variable shuntcapacitor 26l. Use of this filter will give the result shown by thegraph 262 or the graph 263 depending upon the adjustment of the filter.Here the resistor 259 might, for example, have a value of 100,00() ohmsand the capacitor 261i a value of one microfarad. This gives a filtertime constant of 0.1 second so that it takes about 0.1 second to glidefrom one note to another.

Vibnato is a term used to designate primarily frequency modulation of atone. in actual practice, particularly as -the sound reaches the ear, itinvolves frequency and amplitude modulation or waveform modulation orthe combination of all three. lt is used in singing and in playing:certain musical instruments such as the violin and the trombone.

Tremelo, strictly speaking, is a term used to designate amplitudemodulation only. Such modulation, if done by a modulating signal havinga single frequency, does not give a pleasing effect. lt does not soundlike a vibrato. For example, if a violin tone is amplitude modulated bya seven cycle per second sine wave signal, the result will notcorrespond to a vibrato.

We have found, however, that a vibrato effect can be `obtained by meansof amplitude modulation providing a suitable Wave rich in harmonics isused as the modulating wave.

It is well known that a carrier wave that is frequency modulated by asine wave signal consists of a component at 'the carrier frequency andof many side band components. It is also true that if a carrier wave isamplitude modulated by a non-sinusoidal signal having many frequencycomponents, the resulting signal consists of a componentat the carrierfrequency and of many side band components, specifically, a side bandcomponent for each modulating frequency component.

The difference between a frequency modulated signal and an amplitudemodulated signal having many side bands as above described is in thephase relation of the side bands. The human ear, however, is no-tsensitive to phase relations. Thus it appears that it should be possibleto obtain a true vibrato effect so far as the ear is concerned by meansof suitable amplitude modulation. It has been found in practice thatthis is correct, and that a good vibrato effect can be obtained byamplitude modulating with a sawtooth wave, for example.

actas/a FIG. 3 illustrates one suitable circuit for obtaining a vibratoeffect in accordance wi-th the present invention. it comprises abalanced amplier 264 that includes two pentode amplifier tubes 266 and267. The amplifier circuit itself is conventional and includes asuitable gain control circuit. In the example shown, the input signal isapplied to the second grids of the pentodes 266 and 267 while the firstgrids are used for gain control. Suitable operating bias is applied tothe irst grids through a resistor 268.

Gain control voltage is applied to the first grids from a resistor 269thro-ugh a coupling capacitor 271.

Signal passed through the amplifier 264 is amplitude modulated by asawtooth wave from a sawtooth generator 272 which may be of any suitabletype. In the example shown, generator 272 comprises a capacitor 273-that is charged through an yadjustable resistor 274 from a directcurrent source.

After the capacitor 273 charges up to a certain voltage, a gas tube 276breaks down. The capacitor 273 then discharges through an adjustableresistor 277 and the gas tube 276. 'Ihe break-down voltage of the tube276 may be adjusted by adjusting the negative bias voltage applied toits grid. r1[he charging and discharging rates of capacitor 273 may beadjusted by the resistors 274 and 277, respectively.

The sawtooth wave of generator 272 is applied by way of a couplingcapacitor 27SI and the resistor 269` to the gain control grids of theamplifier 266 and 2167.

Since the modulating sav/tooth Wave is rich in even and odd harmonics,the desired vibrato effect will be obtained in the vibrato unit output.The quality of the vibrato is determined by the particular sawtooth waveform used. This will be better understood from the following discussion.

Consider the analysis of a 333 cycle per second sine wave that isfrequency modulated by a 7 cycle per second sine wave signal with amaximum frequency excursion of a semitone. The resulting'modulatedsignal has the following main components with the relative amplitudesindicated in percentage of the unmodulated carrier amplitude:

333 cycles at 58% 340 cycles and 326 cycles (333i7) at 53% 347 cyclesand 319 cycles (3331-14) at 22% 354 cycles and 312 cyc-les (333:\;21) at6% The equivalent amplitude modulated Wave (except for phase relation)is a 333 cycle per second carrier wave modulated by the following sinewaves with the amplir tudes indicated in percentage of the unmodulatedcarrier amplitude:

7 cycles per second at 106% amplitude 14 cycles per second at 44%amplitude 21 cycles per second at 12% amplitude The resulting modulatedwave has the following components:

333 cycles `at approximately 100% amplitude 33317 cycles at 53%amplitude 333i14 cycles at 22% amplitude 333i21 cycles at 6% amplitudeThe immediately preceding tabulation is strictly correct as to sidebands. As to the carrier component, it is not exactly at 100% amplitudebecause of the overmodulation by 'the 7 cycle signal. Furthermore, itwill be noted that this amplitude of approximately 100% differssubstantially from the 58% amplitude of the carrier cornponent in thecase of the frequency modulated wave. This difference in carriercomponent amplitude has been found to be of no apparent importance. Fromlistening tests it `appears that the important thing is to have thecorrect side band components of the correct amplitude.

In FlG. 3a the graph 2S1'shows 'the Wave form of the above-mentionedamplitude modulating wave having the 7 cycle, 14 cycle, and 21 cyclecomponents of substan- -tially the amplitudes tabulated. These 3components are represented by the graphs 232, 283, and 284,respectively. The amplitudes actually assumed in the graphs are 50%, and12.5% for the 7, 14, and 21 cycle components.

It is apparent that the wave form of the signal 2311, can be fairlyclosely approximated by a sawtooth wave, as indicated by the dottedgraph 285, where the steeper side of the sawtooth occupies aboutone-third of the total sawtooth duration.

FIG. 3b shows the graph 231i repeated in dotted line and also shows anactual sawtooth wave 237 that has been found in practice tosatisfactorily approximate the wave 281. The difference between thesawtooth 287 of FIG. 3b and the sawtooth 286 of FlG. 3a is that sawtooth287 bends over somewhat on the steep side. The wave 287 is one that isreadily generated by a simple circuit and has been found to be asuiciently close approximation to a theoretically correct Wave shapesuch as that shown by graph 281.

Referring again to FIG. 3, here the sawtooth wave 287 is drawn in themore usual way with the wave rising up ward in the positive direction.As previously indicated, the ratio of the rising portion of the sawtoothto the falling portion of the sawtooth may be adjusted by adjustmentofthe resistors 274 and 277.

From the foregoing discussion it will be understood that no oneparticular waveform is required for amplitude modulating the musicaltone being passed through amplier 264 (FIG. 3) to obtain the desiredvibrato effect. A waveform such as shown by graph 281 may, of course, beused but the oscillators and adding circuit required to generate itinvolve considerably more apparatus than is required to generate asawtooth wave.

The precise ratio of one to two of the steep portion to the graduallysloping portion of the sawtooth wave is not essential althoughapproximately this ratio seems to give the best results. It may be notedthat the two extreme limits of a sawtooth have been found to beunsatisfactory. These two limits are: rst, a sawtooth wave where thesteep portion is substantially vertical instead of sloping; second, asymmetrical triangular Wave. The first of these gives a thump each timethe steep portion occurs. The second of these does not give the propersound effect, ap-

parently due to the fact that it does not contain any even harmonicterms.

it should also be pointed out that listening tests show that the amountof modulation by the sav/tooth wave to obtain a satisfactory vibratoeffect should be substantially less than 100%. This is contrary to whatis indicated by the tabulated values previously given where 106%modulation (or approximately 100%) is given for the 7 cycle component.Such 100% modulation of the sawtooth tone wave gives too much soundvariation or vibrato effect.

In practice, it has been found that about 56% modulation by the sawtoothtone wave is satisfactory. Stated differently, the 7 cycle, 14 cycle and21 cycle components modulate the sawtooth carrier wave at about 50%modulation, 22% modulation and 6% modulation, respectively. T'nis valueof 551% modulation by the savvtooth wave is not at all critical. In somecases, for example, instead of 50% modulation, it might be preferred tohave 25% modulation or 75% modulation.

In the tabulations of side bands for comparing frequency modulation withsawtooth amplitude modulation, it was assumed that the carrier was asine wave of 333 cycles per second. In the synthesizer, thecorresponding ca rier that is applied to the vibrato unit amplifier 254(FIG. 3) is, of co-urse, a sawtooth wave. However, it is believed thatthe comparison as made assuming a sine wave carrier is accurate, itbeing remembered that a sav/tooth wave actuanverso ally consists of afundamental sine wave component and various harmonic sine wavecomponents.

What is claimed is:

1. Apparatus for producing a vibrato effect in a musical tone whichcomprises a tone generator for generating said musical tone, means forgenerating a Wave ot sau/tooth type having a periodic frequency equal tothat oi the desired vibrato rate and having both even and odd harmoniccomponents, and means operatively connected to said generating means andto said tone generator for amplitude modulating said tone by said Waveto provide an amplitude modulated tone simulating a tone that isprimarily frequency modulated at said vibrato rate.

2. ln combination, a tone source for providing a tone, a musicsynthesizer channel through which said tone is to be passed, said tonesource being operatively coupled to said channel, amplitude modulatingmeans connected in cascade in said channel, and means operativelyconnected to said amplitude modulating means for applying modulatingsingal of sawtooth wave type to said modulating means such that amixture of frequency and amplitude modulation of said tone is simulatedto produce a vibrato ellect.

3. Apparatus for producing a vibrato etcct in a musical tone whichcomprises a musical tone source for providing said tone, an amplitudemodulating device through which said tone is to be passed, said tonesource being operatively coupled to said device, means for generating anelectrical wave of at least approximately sawtooth waveform that has arepetition frequency equal to the desired vibrato rate, and meansoperatively connecting said generating means to said modulating devicefor applying said wave to said modulating device to amplitude modulatesaid tone in accordance with said Wave whereby substantially a frequencymodulation is simulated.

4. Apparatus for producing a vibrato ellect in a musical tone whichcomprises a tone generator for generating said musical tone, means forproducing an electrical wave of savvtooth type and having a certainperiod, said Wave having a portion changing in amplitude in onedirection and another portion changing in amplitude in the oppositedirection, one of said portions having a duration substantially greaterthan one-half the period of the Wave and substantially less than thefull period or the wave, said period being equal to the desired vibratoperiod, and means operatively connected to said Wave producing means andto said tone generator for amplitude modulating said tone by said wave.

5. In a music synthesizer channel, a vibrato unit connected in cascadein said channel and including a tone generator for generating a tone,said unit comprising an amplitude modulating device operatively coupledto said tone generator for amplitude modulating said tone passedtherethrough, a generator of an electrical Wave of sawtooth type acertain period, said wave having a portion changing in amplitude in onedirection and another portion changing in amplitude in the oppositedirection, one of said portions having a duration substantially greaterthan one-half the period ofthe Wave and substantially less than the fullperiod of the Wave, said period being egual to the desired vibratoperiod, and means operatively connecting said Wave producing means tosaid modulating device for applying said Wave to said modulating deviceto amplitude modulate said tone as a function of said Wave.

6. ln a music synthesizer channel having a tone source for providing atone, a vibrato unit connected in cascade in said channel, said unitcomprising an amplitude modulating device operatively coupled to saidtone source for amplitude modulating said tone passed ti erethroug'n, asawtooth generator which provides an electrical wave of a certain periodand of at least approximately sau/tooth Waveform, said sawtooth having aportion changing in amplitude in one direction and another portierichanging in amplitude in the opposite direction, one of said sawtoothportions having a duration substantially greater than one-half theperiod of the sawtooth and substantiallyy less than the full period ofthe sawtooth, said period being equal to the desir-ed vibrato period,and means operatively connecting said wave producing means to saidmodulating device for applying said sawtooth wave to said modulatingdevice to amplitude modulate said tone as a function of said sawtoothWave.

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5. IN A MUSIC SYNTHESIZER CHANNEL, A VIBRATO UNIT CONNECTED IN CASCADEIN SAID CHANNEL AND INCLUDING A TONE GENERATOR FOR GENERATING A TONE,SAID UNIT COMPRISING AN AMPLITUDE MODULATING DEVICE OPERATIVELY COUPLEDTO SAID TONE GENERATOR FOR AMPLITUDE MODULATING SAID TONE PASSEDTHERETHROUGH, A GENERATOR OF AN ELECTRICAL WAVE OF SAWTOOTH TYPE ACERTAIN PERIOD, SAID WAVE HAVING A PORTION CHANGING IN AMPLITUDE IN ONEDIRECTION AND ANOTHER PORTION CHANGING IN AMPLITUDE IN THE OPPOSITEDIRECTION, ONE OF SAID PORTIONS HAVING A DURATION SUBSTANTIALLY GREATERTHAN ONE-HALF THE PERIOD OF THE WAVE AND SUBSTANTIALLY LESS THAN THEFULL PERIOD OF THE WAVE, SAID PERIOD BEING EQUAL TO THE DESIRED VIBRATOPERIOD, AND MEANS OPERATIVELY CONNECTING SAID WAVE PRODUCING MEANS TOSAID MODULATING DEVICE FOR APPLYING SAID WAVE TO SAID MODULATING DEVICETO AMPLITUDE MODULATE SAID TONE AS A FUNCTION OF SAID WAVE.